A thermal inkjet printer includes a printhead having an array of nozzles. Each inkjet nozzle comprises a resistor patterned on a substrate using conventional thin-film fabrication procedures. Ink is allowed to flow into the resistor area, whereafter heating the resistor causes the ink to essentially boil and a tiny droplet of ink is "fired" from the nozzle. The printhead is mounted on a cartridge having a supply of ink for replenishing the nozzles as they are fired.
A printer may have a full-width head or may have a scanning head that is caused to move in a direction perpendicular to a paper path in order to print across the width of a sheet of paper. In inkjet technology, a first level of connection from a scanning printhead is made to a flex circuit. Referring to FIGS. 1a and 1b, an inkjet cartridge 10 is shown as including a housing 12 for storing a reservoir of ink. A printhead 14 having nozzle openings 16 is mounted on one side of the cartridge. Drive signals to heat the resistors of the printhead are provided by traces 24 on a dielectric material 22. Raised contact pads 23 are located at the ends of the traces 24 opposite to the printhead. The flex circuit that comprises the dielectric material 22, the raised contact pads 23 and the traces 24 provides a first level of interconnect from outside circuitry to the resistors of the printhead 14.
The second level of interconnect is from the raised contact pads 23 to a flexible interconnect strip having parallel interconnect lines that extend to stationary logic circuitry of the printer. Referring now to FIG. 2, a flexible interconnect strip 26 includes raised bumps, not shown, that are in registration with the raised contact pads on the dielectric material 22 on the housing 12 of the inkjet cartridge. A snap-spring metal member 28 is fixed to a molded-in carriage 30 by engagement with a ledge member 32 on the cartridge. On the side of the flexible interconnect strip 26 that is opposite to the dielectric material 22 is a series of spring pad bumps, not shown, that urge the raised contact areas of the interconnect strip against the raised contact pads of the flex circuit of the cartridge. These spring pad bumps are described in detail in U.S. Pat. No. 4,907,018 to Pinkerpell et al., which is assigned to the assignee of the present application. When the housing 12 is pivoted to a vertical position as shown by arrow 34, the force provided by the snap-spring metal member 28 aids in obtaining proper electrical contact between the flex circuit and the flexible interconnect strip 26.
Also shown in FIG. 2 is a support member 36 having a bore 38. The circumference of the bore 38 acts as a bearing surface against a stationary carriage rod, not shown, along which the carriage is driven to relocate the printhead across the width of a paper on which ink is to be deposited. Also shown is an interposer arm 40 secured in a shaft 42. The function of the interposer arm is related to mechanically triggering certain features of a service station close to which the carriage resides when printing operations are completed.
A thermal inkjet printer sold by Hewlett-Packard under the trademark DeskJet has an array of fifty drop ejectors. Each drop ejector has a thin film resistor having an electrical resistance of approximately 26.8 ohms. A drop firing pulse of a drive signal is approximately 14.8 .mu.J in energy, with a pulse width of 3.25 .mu.sec. A maximum repetition rate is 3.6 KHz. That is, the operating frequency of the printhead is 3.6 KHz. Consequently, the peak instantaneous power for each resistor is 14.8 .mu.J/3.25 .mu.sec=4.55 Watts. It follows that the peak current is (4.55 Watts/26.8 ohms).sup.0.5 =0.41 amps. Returning to FIGS. 1a and 1b, each raised contact pad 23 and its associated trace 24 must therefore be designed for a peak current of 0.41 amps.
At the maximum repetition rate of 3.6 KHz, in which the firing pulses have a period of 277 .mu.sec, the average current per drop ejector is 0.41 amps.times.(3.25 .mu.sec/277 .mu.sec)=0.0048 amps. If the printing requirements are such that all of the fifty drop ejectors fire simultaneously in a "blackout" mode, the total current is (50.times.0.0048 amps)=0.24 amps. Each of the four common contacts of the printer must therefore be designed for a maximum continuous current of (0.24 amps/4)=0.06 amps.
The raised contact pads 23 must be capable of carrying high peak currents and must have a very low contact resistance to the interconnect strip in order to ensure uniform drive currents to the resistors of the multi-nozzle printhead 14. To achieve a low contact resistance, the pads 23 are made as large as feasible and are plated with gold. Therefore, the interconnect structure plays a major role in the overall cost of the inkjet cartridge 10. Since many of the cartridges used in inkjet printers are disposable cartridges, the cost recurs with use of a printer.
Another difficulty with the conventional design described above is that the need for connection at the interface of the cartridge flex circuit and the interconnect strip places constraints on the design of the remainder of the printer system. For example, an accurately located flat surface of several square centimeters is required for the connection, both on the inkjet cartridge and on the carriage of the printer. Another concern is that the flexible interconnect strip 26 of FIG. 2 should be low in cost, but must be capable of repeated flexing as the carriage 30 moves from side to side during the printing process.
An object of the present invention is to provide a scanning head printer in which electrical connections to a head are achieved in a reliable, low cost design.